DMOSFET with a resistance for improving the reverse bias conduction

Abstract: A vertical MOSFET, which can control AC current flowing through a device only by the gate voltage, is obtained. On an n⁺ silicon layer 11 is formed an n⁻ silicon layer 12. Within the n⁻ silicon layer 12 is formed a p-body region 13. Within the p-body region 13 is formed an n⁺ source region. On top of a substrate are formed a source electrode 191 in contact only with the source region 14 and a base electrode 21 in contact only with the p-body region 13. The source electrode 191 and the base electrode 21 are connected to each other through a resistance at the outside. On a channel region 18 is formed a gate electrode 6 through a gate oxide film (insulating film) 16. When the above semiconductor device is in the reverse bias conduction, the exciting current is controlled only by the gate voltage by setting the current flowing from a source terminal S through the resistance to the base electrode 21, the p-body region 13 and the n⁻ silicon layer 12 to be negligibly small as compared with the current flowing from the source terminal S through the source electrode 191 to the n⁺ source region 14, the channel region 18 and the n⁻ silicon layer 12.

Description: 1. Field of the Invention The present invention generally relates to a semiconductor device. More particularly, the present invention relates to a semiconductor device having a vertical MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistors) structure. 2. Description of the Related Arts FIG. 12 is a cross-sectional view illustrating the structure of a conventional n-channel type DMOSFET (Diffused self-alignment MOSFET). In this figure, a semiconductor substrate 125 is composed of two layers, an n + silicon layer 111 and an n - silicon layer 112, and substrate 125 operates as a drain region. By forming a gate electrode 117 on top of the semiconductor substrate 125 with a gate oxide film (insulating film) 116 disposed therebetween and doubly diffusing p-type impurities and n-type impurities using the gate electrode 117 as a common mask, a p-body region 113 and an n + source region 114 are formed. A channel region 118 is defined within the surface region of the p-body region 113. A source electrode 119 is formed all over the top of the semiconductor substrate 125 to be electrically connected to the p-body region 113 and the n + source region 114. A metal drain electrode 120 is formed all over the bottom of the semiconductor substrate 125 so as to be electrically connected to the n + silicon layer 111. The gate electrode 117, the source electrode 119 and the drain electrode 120 are connected to a gate terminal G, a source terminal S and a drain terminal D, respectively. Now, the operational principal of this prior art transistor will be described. First, the case where the transistor is forwardly biased will be described. When a main voltage is applied in such a way that the drain terminal D is high in electrical potential and the source terminal S is low in electrical potential and a forward bias is applied to the gate terminal G, an inversion layer is formed within the channel region 118, and electrons flow from the n + source region 114 through the channel region 118 into the n - silicon layer 112. As a result of this flow of electrons, electronic current (control current) 121 flows and the transistor is turned to the ON state. FIG. 13 is a graph illustrating the relation between drain current I D , i.e., electronic current 121, and drain-source voltage V DS . The details of the electric characteristics when the transistor is in the ON state with forward conduction are illustrated in the first quadrant of FIG. 13. As the voltage V DS increases, the current I D increases and becomes saturated at a certain current value. When gate-source voltage V GS is increased, a characteristic that the saturation amperage increases appears. Second, the case where the transistor is reverse biased will be described. Main voltage is applied in such a way that the drain terminal D is low in electrical potential and the source terminal S is high in electric potential. When the main voltage is lower than a certain voltage (e.g., -0.7V) and a forward bias is applied to the gate terminal G, an inversion layer is formed within the channel region 118, and electrons flow from the n - silicon layer 112 through the channel region 118 into the n + source region 114. As a result of this flow of electrons, electronic current 122 flows and the transistor is turned to the ON state. On the other hand; when the main voltage is higher than a certain voltage (e.g., -0.7V), a parasitic diode 123 formed between the p-body region 113 and the silicon layer 112 operates. This is because the cathode terminal of the parasitic diode 123 is connected to the drain terminal D and the anode terminal of the same is connected to the source terminal S. Therefore, the parasitic diode 123 is forward biased and the transistor is turned to the ON state regardless of the bias voltage of the gate terminal G. Therefore, in the equivalent circuit of the n-channel type DMOSFET illustrated in FIG. 12, a MOSFET 127 and the parasitic diode 123 are connected in parallel to each other as illustrated in FIG. 14. The details of the electric properties when the transistor is ON with a reverse bias are illustrated in the third quadrant of FIG. 13. The conduction characteristics of the parasitic diode 123 are indicated by a curve A in FIG. 13. As illustrated in FIG. 13, the drain current I D can take amperage controlled by the gate-source voltage V GS only within the region where the voltage is lower than the voltage defined by the curve A. FIG. 15 is a cross-sectional view illustrating the structure of a conventional n-channel UMOSFET (U-Shaped MOSFET) as known e.g. from document "Proceedings of the 28th Intersociety Energy Conversion Engineering Conference (1993), vol. 1, pp. 1249-1254. The n-channel UMOSFET has the same forward and reverse conduction characteristics as those of the n-channel DMOSFET illustrated in FIG. 12, and has the same electric characteristics and equivalent circuit illustrated in FIG. 13 and FIG. 14, respectively. With respect to the electric characteristics of conventional DMOSFETs and UMOSFETs in the reverse biased state, the drain current I D is defined by the parasitic diode characteristics regardless of the gate-source voltage V GS with respect to the drain-source voltage V DS higher than the forward ON voltage (threshold) of the parasitic diode 123 (e.g., 1V or more). As described above, in the DMOSFET and UMOSFET illustrated in FIGs. 12 and 15 respectively, the current I D cannot be controlled by the bias voltage of the gate terminal with respect to the drain-source voltage V DS higher than the forward ON voltage of the parasitic diode 123 (e.g., 1V or more) and therefore, electric conduction can not always be prevented. For this reason, when a large reverse-bias is applied, the circuit positioned downstream therefrom is subject to adverse effects. Therefore, to prevent such adverse effects in circuits subjected to reverse bias, a blocking diode 128 is connected in series to the drain terminal D as illustrated in FIG. 16 so that the blocking diode 128 can be reverse biased to forcedly block the conduction when main voltage is applied so that the anode terminal A is low in electric potential and the source terminal S is high in electric potential. The details of the electric characteristics when the blocking diode 128 is connected are illustrated in FIG. 17. The forward electric characteristics illustrated in FIG. 17 are the electric characteristics illustrated in FIG. 13 but shifted to the right by the amount of the forward voltage drop V AD (e.g., 0.7V) of the blocking diode 128. However, in the semiconductor device illustrated in FIG. 16, a problem exists in that reverse bias signals can be blocked, but cannot be switched. For example, to switch AC signals, a pair of circuits illustrated in FIG. 16 must be connected so as to be reverse parallel to each other as illustrated in FIG. 18. Therefore, in the circuit illustrated in FIG. 18, a problem exists that the number of parts and components, as well as the volume taken up by the device, are doubled. There is also a problem that due to the voltage drop at forward conduction, the forward voltage drop V AD (e.g., 0.7V) of the blocking diode 128 is added to the voltage drop of the MOSFET 127, resulting in increase in voltage loss. Document JP-A-1-135 072 discloses a semiconductor device with a vertical MOSFET structure comprising a semiconductor substrate with a low resistance and a high resistance semiconductor layer on which a body region of an opposite conductivity type is disposed. A low resistance source region is disposed within the body region, while a gate electrode is located on the surface of the body region with an insulating film disposed therebetween for forming a channel region between the high resistance semiconductor layer and the source region. However, since the drain current can be controlled only up to the voltage of the parasitic diode defined by the semiconductor substrate and the body region adverse effects occur to the circuit, when applying a large reverse bias. Moreover, document DE-A-38 24 836 refers to a semiconductor device having an IGBT structure which uses a resistor layer provided between the source electrode connected to the body region and the source region in order to prevent a latch-up effect within the P-N-P-N structure of the IGBT. Document JP-A-59-74 674 discloses a semiconductor device having a vertical n-channel MOSFET structure showing an n + npn + structure. In order to reduce the output capacitance in the n-channel MOSFET the p region is divided into a p - region including two p regions which are separated from each other. However, this document is silent concerning the use of a respective current control means for compensating the adverse effects of the parasitic diode in the PN junction. Finally, document EP-A-0 399 530, on which the preamble of the present independent claim 1 is based, discloses in its figure 9 a vertical MOSFET comprising a semiconductor substrate, a body region disposed on the semiconductor substrate, a source region disposed within the body region, and a gate electrode for forming a channel region between the source region and the semiconductor substrate. An interposing layer serving as a current controlling means is located between the source electrode and the body region in order to prevent breakage of the D MOSFET. In contrast thereto, it is an object of the present invention to provide a vertical MOSFET which can be controlled with respect to both forward and reverse bias and which does not increase the voltage loss without increasing the number of parts and components as well as the volume of the device. According to the present invention this object is achieved by the measures indicated in the present independent claim 1. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, features and characteristics of the present invention will be appreciated from a study of the following detailed description, the appended claims, and drawings, all of which form a part of this application. In the drawings: FIG. 1 is a cross-sectional view illustrating the first embodiment of an n-channel type DMOSFET according to the present invention; FIG. 2 is a graph illustrating the electric characteristics of the n-channel type DMOSFET of FIG. 1; FIG. 3 is a circuit diagram illustrating the equivalent circuit of the n-channel type DMOSFET of FIG. 1; FIG. 4 is a cross-sectional view illustrating the second embodiment of an n-channel type DMOSFET according to the present invention; FIG. 5 is a cross-sectional view illustrating the third embodiment of an N-channel type UMOSFET according to the present invention; FIG. 6 is a cross-sectional view illustrating the fourth embodiment of an N-channel type UMOSFET according to the present invention; FIG. 7 is a cross-sectional view illustrating the fifth embodiment of an n-channel type DMOSFET according to the present invention; FIG. 8 is a cross-sectional view illustrating the sixth embodiment of an n-channel type UMOSFET according to the present invention; FIG. 9 is a cross-sectional view illustrating an applied embodiment of an n-channel type DMOSFET according to the present invention; FIG. 10 is a cross-sectional view illustrating another applied embodiment of an N-channel type DMOSFET according to the present invention; FIG. 11 is a cross-sectional view illustrating still another applied embodiment of an n-channel type UMOSFET according to the present invention; FIG. 12 is a cross-sectional view illustrating a conventional n-channel type DMOSFET; FIG. 13 is a graph illustrating the electric characteristics of conventional N-channel type DMOSFETs and conventional n-channel type UMOSFETs; FIG. 14 is a circuit diagram illustrating the equivalent circuit of conventional n-channel type DMOSFETs and conventional n-channel type UMOSFETs; FIG. 15 is a cross-sectional view illustrating a conventional N-channel type UMOSFET; FIG. 16 is a circuit diagram illustrating the equivalent circuit of a conventional n-channel type DMOSFET and UMOSFET provided with a blocking diode connected to the outside thereof; FIG. 17 is a graph illustrating the electric characteristics of a conventional n-channel type DMOSFET and UMOSFET provided with a blocking diode connected to the outside thereof; and FIG. 18 is a circuit diagram illustrating the equivalent circuit of two pairs of conventional n-channel type DMOSFETs and UMOSFETs connected in reversely parallel to each other and provided with blocking diodes connected to the outside thereof. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [First Embodiment] The first embodiment of a semiconductor device according to the present invention will be described with reference to the appended figures. FIG. 1 is a cross-sectional view illustrating an embodiment of the present invention used in an n-channel type DMOSFET 100, which is a power semiconductor device. In this embodiment, "n-type" is used as the first conductivity type, and "p-type" is used as the second conductivity type. Schematically, in the n-channel DMOSFET 100 of this embodiment, an n - silicon layer 12 (high-resistance semiconductor layer) is formed on an n + silicon layer 11 (low-resistance semiconductor layer), and these two silicon layers 11 and 12 compose a semiconductor substrate 25. Within the n - silicon layer 12 is formed a p-body region 13 (first semiconductor region). Within the p-body region 13 is formed an n + source region 14 (second semiconductor region). On top of the silicon substrate 25 are formed a source electrode 191 electrically in contact only with the n + source region 14 and a base electrode 21 electrically in contact only with the p-body region 13. The base electrode 21 is connected to a base terminal B. The base terminal B is connected to a source terminal S through a resistance 26. On top of the silicon substrate is formed a gate electrode 17, with a gate oxide film (insulating film) 16 disposed therebetween. Next, description will be given concerning the operational principle of this embodiment of the semiconductor device described above. First, the case of forward conduction will be described, which is the same as the case with the above-described n-channel type DMOSFET. Main voltage is applied to the semiconductor device in such a way that a drain terminal D can be at a high potential while the source terminal S can be at a low potential. At this time, as the PN junction between the p-body region 13 and the n - silicon layer 12 is reverse biased, no current flows through a parasitic diode 23 formed between the p-body region 13 and the n - silicon layer 12. There is no voltage drop caused by the resistance 26, and the source potential is applied to the p-body region 13. When a positive bias is applied to a gate terminal G, an inversion layer is formed within the channel region 18 and electrons flow from the n + source region 14 through the channel region 18 into the n - silicon layer 12, so that the n-channel type DMOSFET 100 is turned to the ON state. FIG. 2 is a graph illustrating characteristics between the drain current I D and the drain-source voltage V DS . The details of the electric characteristics when the transistor is ON in the forward conduction is illustrated in the first quadrant of FIG. 2. As the voltage V DS increases, the current I D increases and becomes saturated at a specific amperage. On the other hand, as the voltage gate-source V GS is increased, the saturation amperage rises simultaneously. Next, the case of reverse conduction will be described. When main voltage is applied so that the drain terminal D can be low in electric potential and the source terminal S can be high in electric potential and a positive bias is applied to the gate terminal G, an inversion layer is formed within the channel region 18, and electrons flow from n - silicon layer 12 at the drain side through the channel region 18 into the n - source region 14, and the n-channel type DMOSFET 100 is turned to the ON state. Consequently, the electric potential of the p-body region 13 near the channel region 18 is fixed to the drain potential. Here, when the main voltage is higher than a certain voltage (e.g., 0.7V), the parasitic diode 23 formed between the p-body region 13 and the n - silicon layer 12 operates at locations off the channel region. This is because the cathode terminal of the parasitic diode 23 is connected to the drain terminal D and the anode terminal of the diode 23 is connected to the source terminal S through the p-body region 13. Therefore, the parasitic diode 23 is forward biased and this semiconductor device is turned ON. Therefore, an equivalent circuit of the n-channel type DMOSFET 100 illustrated in FIG. 1 can be shown by the circuit illustrated in FIG. 3, where the parasitic diode 23 and the resistance 26 are connected in series and a MOSFET 27 is connected in parallel thereto. However, as the resistance 26 is set to be very large, the exciting current of the parasitic diode 23 can be restrained to a low, negligible value as compared with the exciting current of the MOSFET 27 (electronic current flowing through the channel region 18 into the n + source region 14). The details of the electric properties when this semiconductor device is ON in a reverse conduction state are illustrated in the third quadrant of FIG. 2. In this embodiment, the exciting characteristics of the parasitic diode 23 illustrated in the third quadrant of FIG. 13 are blocked by the resistance 26, and with respect to reverse conduction, the characteristics only of the MOSFET 27 appear as illustrated in the third quadrant of FIG. 2. Therefore, the semiconductor device can securely be turned ON and OFF by the gate-source voltage V GS . Furthermore, the increase in voltage drop as illustrated in the first quadrant of FIG. 17 does not occur in this embodiment. Therefore, in this embodiment, the current flow through the device can be controlled also with respect to the reverse bias, whereby AC signals can be controlled only by a single n-channel type DMOSFET. Here, in this embodiment, such a semiconductor device as illustrated in FIG. 1 is used to describe the basic composition. However, for actual use as a power semiconductor device, thousands of the unit cells illustrated in FIG. 1 are connected in parallel. At this time, it is preferable that a single resistance 26 is used per whole device or per any given block. It is a matter of course, however, that the resistance 26 can be used per unit cell. Also the resistance 26 can be formed within other regions on the semiconductor substrate or an external resistance can be connected. [Second Embodiment] The second embodiment of a semiconductor device according to the present invention will be described with reference to the appended figures. FIG. 4 is a cross-sectional view illustrating an embodiment of a n-channel type DMOSFET 100 in which the resistance in the first embodiment is formed within a p-body region 13. Schematically, in this embodiment of the n-channel type DMOSFET 100, on an n + silicon layer 11 (low-resistance semiconductor layer) is formed an n - silicon layer 12 (high-resistance semiconductor layer), and these two layers 11 and 12 make up a semiconductor substrate 25. Within the n - silicon layer 12 is formed a p-body region 13 (first semiconductor region), and within the p-body region 13 are formed an n + source region 14 (second semiconductor region) and a p - base region 15 (third semiconductor region). On the substrate 25 are formed a source electrode 19 and a gate electrode 17, the latter being formed through a gate oxide film (insulating film) 16. Next, description will be given to the operational principle of the semiconductor device described above. First, the case of forward conduction is the same as the case with the n-channel type DMOSFET illustrated in FIG. 1. When a main voltage is applied in such a way that the drain terminal D can be high in electric potential and the source terminal S can be low in electric potential and a positive bias is applied to the gate terminal G, an inversion layer is formed within a channel region 18 and electronic current flows from n + source region 14 through the channel region 18 into the n - silicon semiconductor layer 12, so that the n-channel type DMOSFET is turned to be in the ON state. The relational characteristics between the drain current I D and the drain-source voltage V DS are the same as those illustrated in the first quadrant of FIG. 2. Next, the case of reverse conduction will be described. When a main voltage is applied in such a way that the drain terminal D is low in electric potential and the source terminal S is high in electric potential and a positive bias is applied to the gate terminal G, an inversion layer is formed within the channel region 18 and electronic current flows from the n - silicon layer 12 through the channel region 18 into the n + source region 14, and therefore the n-channel type DMOSFET 100 is turned to be in the ON state. When the main voltage is higher that a certain voltage (e.g., 0.7V), a parasitic diode 23 formed between the p-body region 13 and the n - silicon layer 12 operates. The equivalent circuit of the n-channel type DMOSFET 100 illustrated in FIG. 4 is shown by a circuit in which the parasitic diode 23 and the resistance 261 of the p - base region 15 are connected in series and the MOSFET 27 is connected in parallel thereto as illustrated in FIG. 3. Here, the concentration of impurities within the p - base region 15 is set to be very low, so that the resistance 261 is set to be very large. Therefore, the exciting current of the parasitic diode 23 can be restrained to be negligibly small as compared with the exciting current of the MOSFET 27 (electronic current flowing through the channel region 18 into the n + source region 14). The details of the electric characteristics when the semiconductor device is ON with reverse conduction are illustrated in the third quadrant of FIG. 2 as in the first embodiment. The conduction characteristics of the parasitic diode 23 illustrated in the third quadrant in FIG. 2 are blocked by the resistance 261, and with respect to reverse conduction, the characteristics only of the MOSFET 27 appear as illustrated in the third quadrant of FIG. 2. That is, the drain current I D flowing through the device can securely be turned ON and OFF by the gate-source voltage V GS . In addition, increases in the voltage drop, as seen in the first quadrant of FIG. 17, do not occur. Therefore, also in this embodiment, the current flowing through the device can be controlled with respect to reverse bias, whereby AC signals can be controlled only by a single n-channel DMOSFET. Also in this embodiment, when the device is actually used as a power semiconductor device, thousands of the unit cells illustrated in FIG. 4 are connected in parallel. [Third Embodiment] The third embodiment of a semiconductor device according to the present invention will be described with reference to the appended figures. FIG. 5 is a cross-sectional view of an n-channel type UMOSFET 101 to which a power semiconductor device according to the present invention is applied. Schematically, in this embodiment of the n-channel type UMOSFET 101, on the n - silicon layer 12 is formed a p-body region 13, and within the p-body region 13 is formed an n + source region 14. A trench of a gate oxide film (insulating film) 16 is formed extending from the top side of a substrate 25 to the n - silicon layer 12, and through the gate oxide film (insulating film) 16 is formed a gate electrode 17. On top of the substrate 25 are formed a source electrode 191 electrically in contact only with the n + source region 14 and a base electrode 21 electrically in contact only with the p-body region 13. The base electrode 21 is connected to a base terminal B, and the base terminal B is connected to a source terminal S through a resistance 26. Now description will be given to the operational principle of this embodiment. The equivalent circuit of this embodiment of the n-channel type UMOSFET 101 is the same circuit as that of the n-channel type DMOSFET of FIG. 1, with the circuit illustrated in FIG. 3. The electric characteristics thereof are the same as those illustrated in FIG. 2. Accordingly, also in this embodiment, the conduction characteristics of a parasitic diode 23 are blocked by the resistance 26 connected to the outside as in the first embodiment. In this embodiment, the drain current flowing through the device can be controlled by the gate voltage also with respect to a reverse bias without increasing the current loss. Therefore, also in this embodiment, an n-channel type UMOSFET which can control AC signals with the minimum number of parts and components can be obtained. [Fourth Embodiment] The fourth embodiment of a semiconductor device according to the present invention will be described with reference to the appended figures. FIG. 6 is a cross-sectional view illustrating an n-channel type UMOSFET 101 in which the resistance used in the third embodiment is formed within a p-body region 13. Schematically, in this embodiment of the n-channel type UMOSFET 101, on an n - silicon layer 12 is formed a p-body region 13, and within the p-body region 13 are formed an n + source region 14 and a p - base region 15. A trench of a gate oxide film (insulating film) 16 is formed extending from the top side of the substrate to the n - silicon layer 12, and through the gate oxide film (insulating film) 16 is formed a gate electrode 17. Next, the operational principle of this embodiment will be described. The equivalent circuit of this embodiment of the n-channel type UMOSFET 101 is the same as the circuit shown in FIG. 3, which is the circuit for the n-channel type DMOSFET illustrated in FIG. 1. The electric characteristics thereof are the same as those illustrated in FIG. 2. Accordingly, also in this embodiment, the conduction characteristics of a parasitic diode 23 are blocked by the resistance 261 formed in the p - base region 15 as in the second embodiment. In this arrangement, the drain current flowing through the device can be controlled by the gate voltage also with respect to a reverse bias without increasing the current loss. Therefore, also in this embodiment, an n-channel type UMOSFET which can control AC signals without increasing the number of parts and components can be obtained. [Fifth Embodiment] The fifth embodiment of a semiconductor device according to the present invention will be described with reference to the appended figures. FIG. 7 is a cross-sectional view illustrating an n-channel type DMOSFET 100 which is the fifth embodiment of a semiconductor device according to the present invention. Schematically, in this embodiment of the n-channel type DMOSFET 100, a semiconductor substrate 25, a p-body region 131 and an n + source region 141 are hexagonal system SiC single crystals. Here, the azimuth of the surface of the semiconductor substrate 25 is the (0001) plane, and on this plane is formed a p-body region 131 by epitaxial growth. A Ni source electrode 30 electrically in contact with and common to the p-body region 131 and the n + source region 141 is formed using nickel ("Ni"). In this arrangement, the contact resistance of the part where the p-body region 131 is in contact with the Ni source electrode 30 is set to be larger than that of the part where the n + source region 141 is in contact with the Ni source electrode 30, and a resistance 262 is substantially formed in the part where the p-body region 131 is in contact with the Ni source electrode 30. Accordingly, the equivalent circuit of the n-channel type DMOSFET 100 illustrated in FIG. 7 is the same as that illustrated in FIG. 3 due to the resistance 262 in the part where the p-body region 131 is in contact with the Ni source electrode 30. Also in this embodiment, the conduction characteristics of the parasitic diode 23 illustrated in the third quadrant of FIG. 2 are blocked by the resistance 262, and with respect to reverse conduction characteristics illustrated in the third quadrant of FIG. 2, the characteristics only of the MOSFET 27 appear as in the first embodiment. In this arrangement, also with respect to a reverse bias, the drain current flowing through the device can be controlled by the gate voltage without increasing the current loss. As a result, not only AC signals can be controlled only by the gate-source voltage V GS of an n-channel type DMOSFET, but also, as there is no need for an external resistance, the number of parts and components can be reduced. Furthermore, as the structure except for the semiconductor material is the same as that of conventional MOSFETs, the time required for design, for example, is not more than the time required for conventional MOSFETs. [Sixth Embodiment] The sixth embodiment of a semiconductor device according to the present invention will be described with reference to the appended figures. FIG. 8 is a cross-sectional view illustrating the sixth embodiment of an n-channel type UMOSFET 101 according to the present invention. Schematically, in this embodiment of the n-channel type UMOSFET 101, a semiconductor substrate 25, a p-body region 131 and an n + source region 141 are hexagonal system SiC single crystals as in the fifth embodiment. Here, the azimuth of the surface of the semiconductor substrate 25 is on the (0001) plane, and on this plane is formed a p-body region 131 by epitaxial growth. A trench of a gate oxide film (insulating film) 16 is formed extending from the top side of the substrate to the n - SiC layer 28, and through the gate oxide film (insulating film) 16 is formed a gate electrode 17. In addition, a Ni source electrode 30 electrically in contact with and common to the p-body region 131 and the n + source region 141 is formed using nickel ("Ni"). In this arrangement, the contact resistance of the part where the p-body region 131 is in contact with the Ni source electrode 30 is set to be larger than that of the part where the n + source region 141 is in contact with the Ni source electrode 30, and a resistance 262 is substantially formed within the part where the p-body region 131 is in contact with the Ni source electrode 30. Accordingly, the equivalent circuit of the n-channel type UMOSFET illustrated in FIG. 8 is substantially the same as that illustrated in FIG. 3 due to the resistance 262 within the part where the p-body region 131 in contact with the Ni source electrode 30, and the electric characteristics thereof are the same as those illustrated in FIG. 2. Also in this embodiment, the conducting characteristics of the parasitic diode 23 illustrated in the third quadrant of FIG. 2 are blocked by the resistance 262, and with respect to reverse conducting characteristics illustrated in the third quadrant of FIG. 2, the characteristics only of the MOSFET 27 appear as in the first embodiment. As a result, not only can AC signals be controlled using only the gate-source voltage V GS of an n-channel UMOSFET 101, but also as there is no need for an external resistance, the number of parts and components can be reduced. Furthermore, as the structure except for the semiconductor material is the same as that of conventional MOSFETs, increases in the time required for design, for example, can be avoided. As described in the first through sixth embodiments above, the source electrode is divided into two source electrodes. One is a source electrode electrically in contact only with the n + source region, and the other is a base electrode electrically in contact only with the p-body region. The base electrode is connected to the source terminal S through a resistance, or otherwise the source electrode is provided so that the resistance of the part where the p-body area is in contact with the source electrode is set to be larger than that of the part where the n + source region is in contact with the source electrode to have a resistance substantially formed within the part where the p-body region contacts the source electrode. Accordingly, the conducting characteristics of the parasitic diode are blocked by the resistance, and with respect to the reverse conducting characteristics of the semiconductor device, the characteristics only of the MOSFET appear. As a result, the semiconductor device can securely be turned ON and OFF by the gate-source voltage even if the semiconductor device is reverse biased. In this arrangement, AC signals can be controlled only by a single semiconductor device. Furthermore, according to the present invention, increase in voltage drop in forward conduction characteristics does not occur. It should be noted here that the resistance used in the first and third embodiments may either be connected to the outside or buried inside as illustrated in FIG. 9 (in this case, although a resistance layer is formed within the p-body region, the resistance layer may be formed within any other separate region on the semiconductor substrate), for example. In addition, as described above, when the semiconductor device is actually used as a power semiconductor device, thousands of the unit cells illustrated are connected. Therefore, when the resistance is connected to the outside, the resistance may be connected for each unit cell. However, it is preferable that one resistance should be used for the whole device or in any block unit for higher productivity. Furthermore, in the second and fourth embodiments, the base layer 15 is set to be p - type. However, the base layer 15 may be set to be a compensation type (intrinsic type) or a non-doped layer to further increase the resistance therein. If the base layer 15 is set to be an n type or n - type in this structure, an energy barrier is produced between the base layer 15 and the p-body region 13. Therefore, when the base layer 15 of FIG. 4 is set to be an n type or n - type, as shown by n - base layer 152 in FIG. 9, it is preferable that an electrode 192 should be included within the structure as illustrated in FIG. 9, for example. On the other hand, in the fifth and sixth embodiments, the source electrode 30 may be divided into a source electrode 30 and a base electrode 31 as illustrated in FIGs. 10 and 11. In these embodiments, nickel is used as the material of the electrode, which provides high contact resistance to p - type SiC and low contact resistance to n + SiC. However, any material, such as aluminum, may be used as long as the material has the characteristics described above. In addition, the structure of the trench described above is not limited to that illustrated in the figure, and may be U-shaped or V-shaped, for example. Although the above description refers only to the n-channel type, the same effect can be obtained by a p-channel type in which the conductivity type n and p are interchanged. In addition, the present invention is not limited to power MOSFETs, but may be applied to any other type of vertical MOSFET. In the above description, a plurality of positions are disclosed as parts at which the resistance is provided, including the inside of the semiconductor substrate, outside thereof, and the electrode contacting part. However, it is needless to say that various modifications are possible for blocking the conducting characteristics of a parasitic diode, and these modifications are included within the scope of the claims.

Claims: A semiconductor device having a vertical MOSFET structure, comprising: a semiconductor substrate (25) on a drain terminal (20) having a low-resistance semiconductor layer (11) of a first conductivity type and a high-resistance semiconductor layer (12) of the first conductivity type disposed on said low-resistance semiconductor layer (11); a body region (13) of a second conductivity type disposed on said semiconductor substrate (25), a PN junction being defined between said body region (13) and said semiconductor substrate (25); a source region (14) of the first conductivity type disposed within said body region (13) and having a resistance lower than that of said high-resistance semiconductor layer (12); a gate electrode (17) located on a surface of said body region (13) with an insulating film (16) disposed between said gate electrode (17) and said body region (13) for forming a channel region (18) near said surface of said body region (13) between said high-resistance semiconductor layer (12) and said source region (14); characterized in that a source electrode (191; 19) contacts said source region (14); and a current controlling means (26), comprising a resistor (26), connecting said body region (13) to said source electrode (191), wherein said resistor (26) has a resistance value which controls a current flowing in a forward direction through a parasitic diode (23) formed by said PN junction to be smaller than a control current flowing through said channel region (18). The semiconductor device according to claim 1, characterized by a base electrode (21) contacting said body region (13), wherein said resistor (26) of said current controlling means (26) includes an external resistor (26) connected between said base electrode (21) and said source electrode (191) for controlling said parasitic diode current to be smaller than said control current flowing through said channel region (18) during said second mode. The semiconductor device according to claim 1, characterized in that said current controlling means (261) includes a base region (15) which is a semiconductor diffusion layer formed of one of the second conductivity type and a compensated type such that a resistance of said semiconductor diffusion layer is higher than a resistance of said body region (13), said base region (15) being formed in said body region (13), and wherein said source electrode (19) contacts both said source region (14) and said base region (15). The semiconductor device according to claim 1, characterized in that a trench is formed penetrating said body region (13) and reaching said semiconductor substrate (25), and wherein said source region (14) is formed on a side face of said trench within said body region (13) with a distance to said semiconductor substrate (25), said channel region (18) being defined by said distance. The semiconductor device according to claim 1, characterized in that said resistor (26) of said current controlling means (261) includes a contact resistance formed between said body region (13) and said source electrode (30), said source electrode (30) contacting both said body region (13) and said source region (14) and being made of a material making, in association with said body region (13), the contact resistance between said body region (13) and said source electrode (30) higher than a contact resistance formed between said source region (14) and said source electrode (30). The semiconductor device according to claim 5, characterized in that said semiconductor substrate (25), said body region (13) and said source region (14) are made of silicon carbide and said source electrode (30) is selected from the group consisting of nickel, aluminum, nickel alloy and aluminum alloy. The semiconductor device according to claim 6, characterized in that said semiconductor substrate (25), said body region (13) and said source region (14) are made of hexagonal system single crystal silicon carbide. The semiconductor device according to claim 5, characterized in that said source electrode (30) includes a first electrode portion contacting said source region (14) and a second electrode portion contacting said body region (13), and said second electrode portion is made of a material making, in association with said body region (13), a contact resistance between said body region (13) and said second electrode portion higher than a contact resistance formed between said source region (14) and said first electrode portion.

Inventors: HARA KUNIHIKO, MIYAJIMA TAKESHI, TOKURA NORIHITO, HARA, KUNIHIKO, MIYAJIMA, TAKESHI, TOKURA, NORIHITO
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1994
Main classification: H01L29
IPC codes: H01L29